Electromagnetic Radiation GLOBAL ENERGY BUDGET - 1 Electromagnetic Radiation

Earth’ Surface Temperature Amount of incident sunlight Reflectivity of planet Greenhouse Effect Absorb outgoing radiation, reradiate back to surface Clouds Feedback loops Atmospheric water vapor Extent of snow and ice cover

The “Goldilocks Problem” Venus – 460 C (860 F) - TOO HOT Earth – 15 C (59 F) - JUST RIGHT Mars – -55 C (-67 F) - TOO COLD

The “Goldilocks Problem” Temperature depends on Distance from the Sun AND Greenhouse effect of its atmosphere Without Greenhouse effect Earth’ surface temperature 0 C (32 F)

Global Energy Balance - Overview How the Greenhouse Effect works Nature of EMR Why does the Sun emit visible light? Why does Earth emit infrared light? Energy Balance – incoming & outgoing Calculate magnitude of greenhouse effect Effect of atmospheric gases & clouds on energy Why are greenhouse gases “greenhouse gases”?

Global Energy Balance - Overview Understand real climate feedback mechanisms estimate the climate changes that occur Current Future

ELECTROMAGNETIC RADIATION 50% of Sun’s energy in the form of visible light

EMR Self-propagating electric and magnetic wave Moves at a fixed speed similar to a wave that moves on the surface of a pond Moves at a fixed speed 3.00 x 108 m/s

ELECTROMAGNETIC WAVE 3 characteristics speed wavelength frequency or  = c /  The longer the wavelength, the lower the frequency The shorter the wavelength, the higher the frequency

PHOTONS EMR behaves as both a wave and a particle General characteristic of matter Photon – a single particle or pulse of EMR Smallest amount of energy able to be transported by an electromagnetic wave of a given frequency Energy (E) of a photon is proportional to frequency E = h = hc /  where h is Plank’s constant and h = 6.626 x 10-34 J-s (joule-seconds)

PLANK’S CONSTANT E = h = hc /  High-frequency (short-wavelength) photons have high energy Break molecules apart, cause chemical reactions Low-frequency (long-wavelength) photons have low energy Cause molecules to rotate or vibrate more

ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC SPECTRUM Infrared (IR) Radiation 40% of Sun’s energy 0.7-1000 m (1 m = 1 x 10-6 m) Visible Radiation / Visible Light / Visible Spectrum 50% of Sun’s energy 400-700 nm (1nm = 1 x 10-9 m) red longest, violet shortest Ultraviolet (UV) Radiation 10% of Sun’s energy 400-10 nm X-Rays & Gamma Rays – affect upper atmosphere chemistry

EMR & CLIMATE Visible & Infrared most important Ultraviolet Why? Sun? Earth? Ultraviolet Drives atmospheric chemistry Lethal to most life forms

FLUX Flux (F) – the amount of energy (or number of photons) in an electromagnetic wave that passes  through a unit surface area per unit time

Flux & Earth’s Climate

The Inverse-Square Law If we double the distance from the source to the observer, the intensity of the radiation decreases by a factor of (½)2 or ¼

The Inverse-Square Law S = S0 (r0 / r)2 where S = solar flux r = distance from source S0 = flux at some reference distance r0

Solar Flux The solar flux at Earth’s orbit = 1366 W/m2 1 AU = 149,600,000 km (average distance from Earth to Sun) Venus and Mars orbit the Sun at average distances of 0.72 and 1.52 AU, respectively. What is the solar flux at each planet?

The Inverse-Square Law Small changes in earth’s orbital shape + inverse-square law + solar flux CAN CAUSE LARGE CHANGES IN EARTH’S CLIMATE

TEMPERATURE SCALES Temperature – a measure of the internal heat energy of a substance Determined by the average rate of motion of individual molecules in that substance The faster the molecules move, the higher the temperature

TEMPERATURE SCALES Celsius - °C Fahrenheit - °F Kelvin (absolute) – K boiling and freezing points of water Fahrenheit - °F mixture of snow & salt and human body Kelvin (absolute) – K The heat energy of a substance relative to the energy it would have at absolute zero Absolute zero – molecules at lowest possible energy state

TEMPERATURE SCALES

TEMPERATURE CONVERSIONS T (°C) = [T(°F) – 32] / 1.8 T(°F) = [T (°C) x 1.8] + 32.

TEMPERATURE CONVERSIONS Convert the following: 98.6 °F to °C 20 °C to °F 90 °C to °F 100 °F to °C

ABSOLUTE TEMPERATURE T(K) = T (°C) + 273.15 0 K (absolute zero) = -273.15 °C Convert the following: 98.6 °F to K 20 °C to K 90 °C to K 100 °F to K

BLACKBODY RADIATION Blackbody – something that emits/absorbs EMR with 100% efficiency at all wavelengths

BLACKBODY RADIATION Radiation emitted by a blackbody Characteristic wavelength distribution that depends on the absolute temperature of the body Plank Function – relates the intensity of the radiation from a blackbody to its wavelength or frequency

BLACKBODY RADIATION CURVE

Blackbody Simulation Blackbody Simulation

WIEN’S LAW The flux of radiation emitted by a blackbody reaches its peak value at wavelength λ max, which depends on the body’s absolute temperature Hotter bodies emit radiation at shorter wavelengths λ max ≈ 2898 , where T is temperature in Kelvin T λ max is the max radiation flux in μm

WIEN’S LAW

WIEN’S LAW Sun’s radiation peaks in the visible part of EMR 2898 / 5780 K ≈ 0.5 μm Earth’s radiation peaks in the infrared range 2898 / 288 K ≈ 10 μm

WIEN’S LAW

ELECTROMAGNETIC SPECTRUM

THE STEFAN-BOLTZMANN LAW The energy flux emitted by a blackbody is related to the fourth power of the body’s absolute temperature F = σ T4 , where T is the temperature in Kelvin and σ is a constant equal to 5.67 x 10-8 W/m2/K4 The total energy flux per unit area is proportional to the area under the blackbody radiation curve

THE STEFAN-BOLTZMANN LAW

THE STEFAN-BOLTZMANN LAW Example: a hypothetical star with a surface temperature 3x that of the Sun Fsun = σ T4 = (5.67 x 10-8 W/m2/K4) (5780 K)4 = 6.3 x 107 W/m2 Fstar = σ T4 = (5.67 x 10-8 W/m2/K4) (3x5780 K)4 = 34 x σ (5780 K)4 = 81 Fsun

THE NATURE OF EMITTED RADIATION Wien’s Law – hotter bodies emit radiation at shorter wavelengths Stefan-Boltzmann Law – energy flux emitted by a blackbody is proportional to the fourth power of the body’s absolute temperature SO – the color of a star (wavelength) indicates temperature, temperature indicates energy flux